Copolymerization of styrene and alpha-methylstyrene



May 22, 1962 COPOLYMERIZATION OF' STYRENE AND ALPHA-METHYLSTYRENE K. W. DOAK Filed Feb. 25, 1958 3,036,053 CPOLYMERIZATON F STYRENE ANI) ALPHA-METHYLSTYRENE Kenneth W. Donk, Pittsburgh, Pa., assigner to Koppers Company, Inc., a corporation of Delaware Filed Feb. 25, 1958, Ser. No. 717,472 Claims. (Cl. m50-88.2)

This invention relates to an improved polymerization process for copolymerizing vinyl aryl compounds. In one specific aspect, it relates to a suspension polymerization process for copolymen'zing styrene and alpha-methylstyrene.

Styrene-alpha-methylstyrene copolymers are of considerable commercial interest because of their enhanced dimensional stability to heat (measured by heat-distortion temperature of Vicat softening point) compared with that of polystyrene. Commercial polystyrene has a-heat distortion temperature of about 93 C. and a Vicat softening point of about 101 C. The increased cost of producing a styrene-alpha-methylstyrene copolymer is more than offset by obtaining a polymeric material having a Vicat softening pointV of S-115 C. Such a material is useful for many purposes wherein polystyrene is unsuitable, e.g. in the manufacture of radio cabinets (for which the Fire Underwriters Laboratories specifications call for a material having a higher softening point than polystyrene) and similar articles.

It is well-known that alpha-methylstyrene will not readily polymerize under the conditions normally used for the free radical polymerization of styrene. Even with the use of large amounts, eg. 2-l0%, of a penoxide in- -itiator such as benzoyl peroxide, a low yield of a very low molecular Weight product is obtained. v i

VIt is likewise known that mixtures of styrene and alpha-methylstyrene do not polymerize in the same manner as styrene alone, since the presence of alpha-methylstyrene has an inhibiting effect on the copolymerization. Thus, in a free radical polymerization, a mixture of styrene and alpha-methylstyrene polymerizes more slowly than styrene alone under identicall conditions. The rate of copolymerization decreases with increasing amounts of alpha-methylstyrene.

In the well-known process of Gaetano F. DAlelio described in U.S. Patent No. 2,656,334, styrene-is polymerized in the presence of a suitable combination of peroxide catalysts under appropriate conditions untily as little as-0.01% of the monomer remains.- If a mixture of styuene andalpha-methylstyrene is polymerized under these same conditions, the alpha-methylstyrene enters the copolymer much more slowly than the styrene; thus the portion of alpha-methylstyrene in the residual monomer mixture increases as a higher percentage conversion of monomer to polymer is effected. Generally above about 90% conversion, the residualmonomer contains a high United States Patent O proportion of alpha-methylstyrene which, as I have pre-p viously indicated, will not polymerize to a high`molecular Weight product. It is well-established that vinyl aryl polymers having substantial amounts of residual monomer and low molecular Weight polymers present therein have certain undesirable properties, viz: high shrinkage in boiling water, low resistance to heat distortion, potential discoloration due to oxidation of the -monomer and crazing due to escape of volatile materials. n j

Accordingly, to obtain a commercially useful product it is necessary to remove the residual monomer and other by-products formed during -the polymerization reaction. If the copolymerization of styrene and alpha-methylstyrene is effected by conventional styrene polymerization techniques, such as that of DAlelio, removal of the undesirable residual materials is economically dubious.

A spaans?. Patented May 22, 1962 .ICC

Moreover,4 the time required for polymerization, using such a process, is commercially prohibitive.

Various procedures have been proposed for overcoming .the aforesaid diiiiculties. One method, that of British Patent 718,687 involves a carefully controlled bulk polymerization with or Without a catalyst followed by a vaporization at pressures under 50 mm. of mercury and temperatures of 24U-280 C. yUnfontunately, this process is somewhat limited, since, in order to obtain a high conversion of monomer to polymer (above and to produce a polymer having a commercially desirable Vicat softening point (10S-115 C), it is necessary to polymerize for -about 10-20 days. Furthermore, in order to make copolymers having a high styrene content, e.g. 75%, in good yield and of high softening point at least l0 days polymerization time is required. Copolymers having a styrene content of 7590% having the aforesaid characteristics, are especially desirable because of reduced raw material cost. It is quite obvious that Iwith a ten-day polymerization time, the production cost of the copolymer is very high.

British Patent 752,572 describes an improved process for making styrene-alpha-methylstyrene copolymers containing l0-30% alpha-methylstyrene wherein `a mixture of ytwo peroxides With different ltemperature optima, e.g. benzoyl peroxide and di-t-butyl peroxide, are used to initiate the polymerization. The monomer mixture is heated in bulk at 60-70 C. for 24-48 hours and kthe temperature is thereafter increased incrementally toa maximum of 190 C. The total polymerization time required is greater than hours. The polymer is then cooled, crushed, and heated ina vacuum at 1Z0-130 C. for up to 30 hours to remove vola-tile materials. While this process is capable of producing material having a Vicat softening point as high as C. with good conversions, the total time required to obtain the ultimate ,product still makes Ithe process commercially unattractive. I Vhave found a method of making styrene and alphamethylstyrene copolymers containing 5-30% -of alphamethylstyrene wherein the polymerization time is markedly reduced, and copolymers having molecular weights up to 150,000 and intrinsic viscosity of 0.5 to 1.0 (which delineates the molecular Weight range) and Vicat softening temperatures of 10S-115 C. are obtained. Using my novel process detrimental side reactions are minimized, so that volatile materials, consisting essentially of residual monomers, are readily removed.

It is, therefore, an object of the present invention to provide a novel method for copolymerizing styrene and alpha-methylstyrene, whereby Ta commercially desirable copolymer is produced and at the same time the polymenization' time required is reduced by about 60S0%'.

In accordance with the present invention I am able to produce a commercial copolymer in a considerably shorter time than has been heretofore possible by increasing the rate of polymerization and concomitantlygcontrolling the concentration of free radicals at all times during-polymerization. The concentration of free radicals is controlled by choosing a particular temperature range, with or without the use of a peroxide initiator, that vwill permit za. slight increase in Vthe concentration of jfree radicals of the system as the temperature increases and polymerization progresses.v i

It is helpful in understanding my invention to keep in mind the following theoretical considerations.M

`If the concentration of free'radicals in the. system is lcontrolled Vby temperature alone, the Arate at which the kth is the rate constant for the thermal initiation of the polymer chain, and

(M) is the total concentration of monomer (styrene and alpha-methyl-styrene) If a peroxide initator is used, the polymerization can be effected at lower temperatures and the free radicals provided by the initiator predominate inthe system. Since the major portion of the free radicals are furnished by the initiator, the rate at which free radicals are for-med in the system, or the rate of initiation, is determined largely by 'Equation 2.

wherein 1 R1 is the rate of initiation,

k, is the rate constant for initiation of a polymer chain,

and (I) is the concentration of initiator.

The rate at which the polymerization progresses is shown in Equation 3. E (3)' Rp=kptR (M) wherein: Y Rp is the rate of polymerization, kp is the rate constant for polymerization or propagation of a polymer chain, `(R) is the free radical concentration, and

(M) is the total concentration of monomer.

The rate at which growing polymer radicals react to terminate the growth of chains is shown below in Equation 4. (4) Rt=2k(R )2 wherein: Y Y

Rt is the rate of termination, kt is the rate constantV for the termina-tion chains, Iand Y Y r (R') is the free radical concentration.

It is apparent from the aboyeequalions that't-he Vrate .'propagation and termination ofthe individual .polymer chains. VAt low degrees of conversion of monomer to polymer, wherein conditions xapproach the steady state,

of poiymer the rates of initiation andY termination are substantially Y equal. Assuming this, Equation 2 can be substituted intoV Equation 4 'as follows:

kfz/citar Y i Thus, thelrate ofrpolymerizlation at low conversions is f6.0

uproportional to Ythe square rootof the initiator concen- Y tration.

The degree of polymerization at Iany temperature is, therefore, inversely proportional to the square root of the initiator concentration- -It is also apparent from Equation 8 that the degree of polymerization is dependent upon the relative values of 1the three rate constants. Each rate constant Varies logarithmically with the temperature. The temperature coecient for the rate of initiation is greater than that for the rate of propagation. The temperature coefficient forV `the rate of 'termination is less than that for the rate of propagation. Y

-t is obvious 'that as the temperature of the system is increased more free radicals will be formed, eitherV by thermal initiation or by the decomposition of a catalyst initiator present in the system. Thus, at any conversion or temperature, if the concentration of initiator is held constant, the rate of polymerization incre-ases and the molecular Weight decreases. If the concentration of free radicals is held constant and the temperature is increased, the rate of polymerization increases less rapidly as the temperature increases, but the molecular weight increases. l have found that by controlling the rate of formation of free radicals, either by choosing an Iappropriate temperature range or by choosing an Yappropriate temperature range coupled with an initiator with the proper rate of decomposition, to permit a slightincrease in the free radical concentration as the temperature increases, the rate of polymenization can be increased and a high molecular weight product can be obtained. By -a sligh increase I mean that increase in the free radical concentration that will result in a product having an intrinsic viscosity of 0.5-1.0 as determined by solubility in toluene at 30 C. My invention can be more clearly understood with refenence to the accompanying drawing wherein:

The FIGURE shows the rate of polymerization for an 85:15 mixture of styrene-alplra-methylstyrene monomers at 80 C. (curve B) and 100"V C. (curve A), and styrene at 100 C. (curve C) using various initiator systems (ie. lauroyl peroxide, benzoyl peroxide, and t-butyl perbenzoate) VVplotted as a Yfunction of the reciprocal of the molecular Weight.

From theY figure it is observed that the copolymerization of styrene and alpha-methylstyrene in the presence of Vt-butyl perbenzoate is `a linearV function of the reciprocal of the molecular weight. This observation is likewise true for the polymerization of styrene at 10.0 C.`usng three different peroxides; thus showing that the only difference in behavior between the various peroxides is their rate of free radical formation, i.e. their rate of decomposition. f The slopes of the curves of the figure are proportional to the term kp2/kt in ,Whichrkp and kt are the rate constants vfor chain Vpropagation and chain termination respectively. VIn` the case of the copolymer it is readily seen that if the rateof free radical formation is A The 'degreefofppolymerization (or molecular weight) is ,Y

' equal torthe rate of propagation divided by the rateof YVtermination.'NDiyidiug 'Equation 3 by Equation 4 gives;V

Substituting Equation Sinto Equation gives:

Atemperatures than is the rate of chain termination. :comparing thevr slopes of the curves -forstyrene-alpha- ^methylstyrenejat C. and styrene at 100 C., it is readily selen'that at 1a given temperature the rate of copt ilyrneriz'ation offthe`85-l5 monomer mixture can be only 30% as great as therate of homopolymerization of styrene it anvequivalent molecular weightV isteV be obtained. ,'Ifhus,V a high Y polymerization temperature (12S-'140 VC.) or a VpolymerizationK Vtemperature of 100--125c C, With-the proper choice of Vinitiator is imperaa reasonable YYpolymerization time. Y

tive/n order Vtop oibtain a high molecular'weight product in Y TABLE 1 Polymerization of 85:15 Mixture of Styrene and Alpha- Methylstyrene in the Presence of t-Butyl Perbenzoate, (l)

Percent Rate Mol. 108 Part I (I) Hours Conv. Percent] Wt. Mol. Hr. X"3 Wt.

Polymerization at 80 C.

Polymerization at 100 C.

Polymerization of Styrene ot 100 C. in the Presence of Various Peroxide Initiators (I) Pts. Percent Rate Mol. 10E Peroxide (I) Hours Conv. percent/ Wt. Mol. Hr. 10s Wt.

t-Butyl Perhenzoate 1. 75 4. 34 2. 48 400 250 0. 040 0. 200 1. 75 8. 18 4. 67 305 328 0. OS2 0.286 l. 75 11.33 6.47 250 460 0.119 0.345 1. 50 11.56 7. 71 225 445r 0.241 0.491 l. 50 15. 87 10. 58 167 599 0. 398 0. 631 1. 50 20. 98 13. 99 142 704 i 0. 563 0. 750 1. 25 21. 29 17.03 120 831 0.802 0.896 1. 25 25. 65 20. 52 105 953 Dicumyl Peroxide t-Butyl Peroxide Curves similar to those in the figure are used to determine the rate of polymerization permis-sible at a given temperature to obtain a product of a. predetermined molecular weight (or intrinsic viscosity).

The method of my invention comprises forming an aqueous suspension of styrene and alpha-methylstyrene monomers, wherein the mixture of monomers consists of 95-70 parts styrene and 5-30 parts alpha-methylstyrene. The suspension is stabilized by adding a suspending agent such as tri-calcium phosphate. The methods of preparing the suspensions, various suitable suspending agents, and amounts used are described in U.S. Patents 2,687,408 and 2,715,118 issued to I. M. Grim. In addition to the sta-V bilizing agent, the use of a small quantity of surface active agent to reduce surface tension is often helpful. Any nonionic, anionic, and cationic surfactant material is acceptable for this purpose. Alkyl aryl sulfonates and octylphenoxy polyether alcohols are quite suitable. Generally speaking, the mixed monomers comprise about 40-50% by weight of the total suspension. The percentages of suspending agent and surfactant added to the system are based on the total weight of monomer. From 0.2-0.5% suspending agent and 0.001-0.05% surfactant are adequate.

The concentration of free radicals present at any time during polymerization may be controlled to providek a slight increase in the concentration thereof in the system by operating the temperatures from 12S-140 C. Somewhat lower temperatures, viz: 100-125 C., may be used in combination with a proper choice and amount of or ganic peroxide initiator.

The choice of a peroxide initiator for purposes Vof my invention is predicated on its decomposition temperature. An eiective initiator Will decompose slowly at the temperatures employed for the polymerization process. Thus, to be effective the organic initiators of the present invention must decompose in the range of about 100125 C. Such peroxides include, but are not limited to tertiarybutyl perbenzoate, dicumyl peroxide, di-t-butyl peroxide, 2,2-bis-t-butyl peroxy-butene, di-tertiary-butyl di-peroxy phthalate, and the like. Other suitable catalysts include those falling within the desired decomposition range which are listed in U.S. Patent 2,676,944 of K. W. Doak and Belgian Patent 547,998. Oatalysts of the type R-O-O-R are preferred to the type R--O-OLH.

The catalyst should be present in amounts ranging from ODO-0.50% based upon the initial weight of the monomer mixture in the suspension. As I have indicated, the catalyst may be omitted altogether using the higher polymerization temperatures of my invention, since a sumcient concentration of free radicals is produced by thermal initiation. If too great an excess of catalyst is employed, the concentration of free radicals becomes too high and large amounts of low molecular weight polymer are formed. 'If the polymer chains are not permitted to grow to a suliicient average length, it is not possible to obtain a iinal product having the desired high Vicat softening point. The free radical concentration used also varies with the intended composition of the polymer, since styrene Iand alpha-methylstyrene polymerize at a different rate. The preferred amount of catalyst for purposes of -my invention ranges from (L10-0.20% by Weight of monomer.

The actual polymerization is conducted in an inert atmosphere. Nitrogen gas is quite suitable for Vthis purpose. The temperature of polymerization varies with the desired composition of polymer, the choice of catalyst, if any, and the amount of catalyst employed. Generally speaking, lower temperatures are used for making the copolymers with higher alpha-methylstyrene content.V

I'have previously indicated that temperatures of about 12S-140 C. are satisfactory when no catalyst is used. When a catalyst is used in the amount indicated hereabove, temperatures of 100125 C. are quite suitable. Often, it is effective to conduct the polymerization at a lower temperature, e.g. 100-115 C., fora maior portion of the polymerization time and thereafter to cornplete polymerization at temperatures of about 13S-140 i C. Polymerization is continued until at least about 90% conversion -is reached -to avoid a costly and time-consuming separation and recovery of residual .monomers from the polylmerized mass. During polymerization the pressure in the reactor increases from about atmospheric to about 35-40 p.s.i.g. v

The time required to complete the polymerization ran'ges from about 11-26 hours when a catalyst initiator is used, depending, ofcourse, upon the alpha-methylstyrene content of the copolymer. For example, if a -30 styrene to alpha-methylstyrene copolymer is desired, the polymerization vtime will run as high asf36 hours. If no catalyst is used, polymerization can generally be completed in 30-50 hours. The catalytic technique is kthe preferred embodiment of my invention, since the saving in polymerization time generally morethan` offsets the cost of the catalyst initiator. Y K

To obtain the superiorrhighV molecular weight, high Vicat softening point copolymers of the present invention, it is necessary to remove Vthe residual vmonomer mixture and other low molecular weight materials'formed during the course of reaction. Often, before removal of these residual materials, Vicat softening temperatures of copolymers made -by my novel method are greater Vthan 100 C. After the residual material is removed, the Vicat softening point ranges from about S-115 C.

The residual material may be removed in any manner conventional to the art, e.g.V evaporation under vacuum, precipitation from a solvent, or steam stripping. Steam stripping at temperatures above 100 C. affords apartic- .ularly rapid way of removing the undesirable residual material. Steam stripping is accomplished by passing superheated steam atY a predetermined temperature through the suspension of polymer particles. The resid- .ual materials are vaporized and carried off with the steam.

Slightly higher temperatures are more effective since, at

these temperatures, the residual materials can-be more rapidly removed. A temperature range of 10D-150 C.

is quite suitable. The time required for steam stripping is about 5-10 hours, if atV least 90% conversion is obtained during the polymerization process.

The eiectiveness of the steam stripping technique as applied'to my novel polymerization process is astonishing in view of findings reported by Daumiller in U.S. Patent 2,713,043. Daumiller noted that steam strippingat 100 C. does not significantly improve polystyrene orV interpolymerizates thereof having a softening point similar to that of polystyrene and prepared from styrene and other organic compounds polymerizable under the same conditions as polystyrene. Y.

I have previously indicated the importance of obtaining at least about-90% conversion before stopping the Y polymerization reaction. The presence of excessive amounts of residual monomer mixture effectively reduces yield of product obtained per run. Furthermore, time required for steam stripping is greatly increased and the cost of recovering the monomer from the steam after it has been removed from the polymer becomes-,exceedingly high. For example, if the conversion is only about-76%,

32 hours steam stripping time is required.

Y As I have previously indicated, the-residualrnaterial may also be removed by evaporation in vacuum. Using this process the-polymeris cooled'and heated in vacuum at temperatures of about 120--130C.`

Precipitation from aV solvent such as toluene or 'ben-V zene is also an effective method of removing residallma'- terial. By thisV method the polymerized ,mass is dissolved in a solvent, e.g. benzene, and then -added to an agitated bath of a non-solvent, e.g.V methanol. VThe polymer precipitates out and the residual material remains in solution.

After removal of the'residual material the polymer thus obtained is treated With 37% HC1, centrifuged, and

v then washed with Water.V The polymer is thereafter dried at a temperature of about 70 C. for-about 2'hours.

YThe copolymers obtained by mynovel method are characterized by Va Vicat softeningpoint Yof about 105- 115 C., a heat distortion temperature of 20S-225 F.,

ian' intrinsic viscosity of about 0.5-1.0, aresidual monomer content of about lessthan labout 0.1%, a notched impact'strength comparable to polystyrene, and a tensile Vstrengthof about 6000-9000.V

The Vicat 'softening point is determinedV according toV a procedure described in Chemistry and Technology Yof '8 in toluene at C. Other measurements are made as follows:

Property: Testmethod used Heat distortion temperature, n 5 F. ASTM-D-648-56 Notched impact strength, ft.-

lb./in. ASTM-D-256-56 'Tensile strength, p.s.i ASTM-D-63S-56T 15 EXAMPLE I A mixture of V90' parts styrene, l0 parts alpha-methylstyrene, 0.12 part t-butyl penbenzoate, 112 parts deionized Water, 1.91 parts calcium phosphate, and 0.0064 part of Nacconol NRSF, a commercially available alkyl aryl sulfonate, Was placed in a 10G-gallon reactor. The reactor Was purged with nitrogen and the mixture was heated to V100 C. during a period of 1.5 hours. The temperature Was then raised to 110 C. and held thereat for 7 hours. It was then raised to 140 C. for 1/2 hour` Steam stripping was started at 140 C. with a reactor pressure of 40 p.s.i.g. After 3.5 hours, 5.8 parts of hydrocarbons had been recovered. The product was then treated with 37% HCl, centrifuged, and thereafter Washed with Water. It was dried at a temperature of about 70 C. for two hours. The product had a Vicat softening temperature of 104 C. The relative viscosity of a 1% solution of the copolymer in toluene at 30 C. was 2.0.

EXAMPLE II 35 A mixture of 85 parts styrene and 15 parts alpha-methylstyrene containing 0.10 part t-butyl perbenzoate, was copolymerized in suspension in a manner substantially similar to that described in Example I. The heatingtime at 110 C. was 15 hours; thereafter Vthe temperature was 40. raised to. 135 C. The unreacted monomers were removed by stripping with steam at 135 C. in about seven hours. The Vicat softening point of the product was 106 C. and the relative viscosity of a 1% solution of the copolymer in toluene at 30 C; was 1.9.

EXAMPLE III Following the method of the previous examples, a mixlture of 70 parts styrene and 30 parts alpha-methylstyrene was copolymerized in suspension in the presence of 0.14

parti-butyl perbenzoate.V At yintervals during the polymerization a sample of the product was removed and separated from the monomer. The polymerization time and temperature, the percent conversion, the intrinsic viscosity, the molecular weight, and the Vicat softening point for each sample are indicated below in Table II.u

TABLE II Copolymerizcrtion of 70:30 Mixture of Styrene and Alpha- Methylstyrene with 0.14 Parts t-Butyl Perbenzoate Hours at Hours at Percent'.V Intrinsic Molecular Vicat 100 C. 110 C. Conver- Viscosity n Weight softening sionV Y Point 1o 32 o. 62 sv, ooo V 20 56 0. 7l 104, 000 109 36 93 0. 83 Y 130, 00D Y 114 50 96 0. 83 130, 000 111 50 Y 12 97 0. 84 132, 000 111 50 l2 l98 In reinen@ at 30 C.v Y 7U Y b Not separated from monomer. Y

' Y EXAMPLE IVv Y. I The Vprocedure of Example III was'substantially re- Y peated. Amixture of 90 parts styrene and 10 parts alphamethylstyrene was copolymerized at 120 C. inthe presp ence of 0.11 part of dicumyl peroxide. A similar monomer mixture was also copolymerized in the absence of a peroxide. Samples were taken at intervals during polymerization and residual monomers were removed therefrom -by precipitation from benzene. The results are summarized Ibelow in Table III.

Following the procedure of Example I, a mixture of 80 parts styrene and 2O parts alpha-methylstyrene was copolymerized in the presence of 0.10 part t-butyl perbenzoate. 'Ihe mixture was heated for 24 hours at 105 C. and then for two hours at 135 C. The residual monomer content was 1.3% styrene and 1.4% alpha-methylstyrene, based upon the weight of total charge, After removal of the monomers by precipitation from benzene, the Vicat softening temperature was 110 C. The relative viscosity of a 1% solution of the copolymer in toluene at 30 C. was 2.1.

EXAMPLE VI A mixture of 85 parts styrene and 15 parts alpha-methylstyrene was copolymerized in suspension in the presence of 0.12 part t-butyl perbenzoate using the procedure of the previous examples. The temperature was held at 110 C. for 15 hours, and then at 120 C. for six hours. The product had a Vicat softening temperature of 101 C. even without the removal of the residual monomers. After steam stripping for seven hours at 135 C. the Vicat softening point was 107 C. The ASTM heat distortion temperature was 210 F. The residual monomer recovered was less than 1% EXAMPLE VII Using the method of the previous examples, a mixture of 85 parts styrene, 15 parts alpha-methylstyrene and 0.20 part of `t-butyl perbenzoate was polymerized in suspension for 11 hours at 100 C. steam stripping was carried out at 100 C. for 30 hours, then lat 150 C. `for two hours. The conversion was 76% and the ASTM heat distortion temperature was 211 F. The relative viscosity of a 1% solution of the copolymer in toluene at 30 C. was 2.0. This example illustrates the necessity of obtaining about 90% conversion in order to obviate a prolonged steam stripping operation `and the requirement of expensive recovery `facilities for recovering the removed monomer yfrom the steam.

I have thus provided a novel, commercially operable method for styrene-alpha-methylstyrene polymerization having a considerably reduced polymerization time. The product made by this process is exceptional in respect of its dimensional stability to heat. The reduction of the polymerization cycle makes possible a substantial increase in plant out-put, thus reducing the overall equipment cost per pound of product. The high conversions obtained with my method obv-iate sizable recovery facilities for processing the residual monomer separated after the polymerization step.

I claim:

l. Method of making styrene-alpha-methylstyrene copolymers by suspension polymerization by suspending in an aqueous medium, in the presence of a suspension agent, a mixture of monomers consisting of 95-70 parts styrene and 5-30 parts alpha-methyl styrene, controlling the free radical concentration throughout the polymerization -to permit a slight increase thereof as polymerization progresses by adding to the mixture 0.10-0.20 parts by weight based upon the weight of said mixture of an organic peroxide decomposing at a temperature of 100- 125 C., heating the catalyst-containing mixture in an inert atmosphere at a temperature of 100-125 C. until at least about 90% conversion of monomer to polymer has lbeen eected, whereby a copolymerhaving an intrinsic viscosity'of 0.5-1.0 is produced, and removing the residual monomer from the polymerized mass.

2. Method according to claim 1 wherein the residual monomer is removed from the polymerized mass by precipitation of the polymer from a solvent medium.

3. Method according to claim 1 wherein the catalyst is selected from the group consisting of t-butyl perbenzoate, dicumyl peroxide, and di-t-butyl peroxide.

4. Method according to claim 3 wherein the catalyst is t-butyl perbenzoate.

5. Method of making styrene-alpha-methylstyrene copolymers having a Vicat softening point of S-115 C. by suspension polymerization by suspending in an aqueous medium, in the presence of a suspension agent, a mixture of monomers consisting of 95-70 parts styrene and 5-30 parts alpha-methylstyrene, controlling the free radical concentration throughout the polymerization to permit a slight increase thereof as polymerization progresses by adding to the mixture 0.10-0.20 part by weight based upon the Weight of said mixture of an organic peroxide decomposing at a temperature of 100-125 C., heating the catalyst-containing mass in an inert atmosphere at a temperature of 10G-125 C. for about 11-26 hours, and removing the residual monomer from the polymerized mixture by stripping with steam at temperatures of 10U-150 C. for 5-10 hours.

References Cited in the tile of this patent UNITED STATES PATENTS FOREIGN PATENTS Germany Jan. 19, 1956 OTHER REFERENCES DAlelio: Fundamental Principles YofA Polymerization, 1952, John Wiley & Sons, New York, N.Y., pages 205- 206. 

1. METHOD OF MAKING STYRENE-ALPHA-METHYLSTYRENE COPOLYMERS BY SUSPENSION POLYMERIZATION BY SUSPENDING IN AN AQUEOUS MEDIUM, IN THE PRESENCE OF A SUSPENSION AGENT, A MIXTURE OF MONOMERS CONSISTING OF 95-70 PARTS STYRENE AND 5-30 PARTS ALPHA-METHYL STYRENE, CONTROLLING THE FREE RADICAL CONCENTRATION THROUGHOUT THE POLYMERIZATION TO PERMIT A SLIGHT INCREASE THEREOF AS POLYMERIZATION PROGRESSES BY ADDING TO THE MIXTURE 0.10-0.20 PARTS BY WEIGHT BASED UPON THE WEIGHT OF SAID MIXTURE OF AN ORGANIC PEROXIDE DECOMPOSING AT A TEMPERATURE OF 100125*C., HEATING THE CATALYST-CONTAINING MIXTURE IN AN INERT ATMOSPHERE AT A TEMPERATURE OF 1000-125*C. UNTIL AT LEAST ABOUT 90% CONVERSION OF MONOMER TO POLYMER HAS BEEN EFFECTED, WHEREBY A COPOLYMER HAVING AN INTRINSIC VISCOSITY OF 0.5-1.0 IS PRODUCED, AND REMOVING THE RESIDUAL MONOMER FROM THE POLYMERIZED MASS. 